195Para-Benzoquinone in DMSO
A solution of para-benzoquinone in DMSO reacts with inorganic cyanide to form a cyanophenol,
which is fluorescent. Illumination with a UV light gives a green/blue glow if the test is positive.
Copper and an Aromatic Amine
As used by fumigators to detect hydrogen cyanide, copper (II) salt and an aromatic amine such as
benzidine is added to the sample; as an alternative to benzidine an alternative amine di-(4,4-bis-
dimethylaminophenyl) methane can be used. A positive test gives a blue color. Copper (I) cyanide
is poorly soluble.
By sequestering the copper(I) the copper(II) is rendered a stronger oxidant. The copper, in a
cyanide facilitated oxidation, converts the amine into a colored compound. The Nernst equation
explains this process. Another good example of such chemistry is the way in which the saturated
calomel reference electrode (SCE) works. The copper, in a cyanide-facilitated oxidation, converts
the amine into a colored compound.
Pyridine-Barbituric Acid Colorimetry
A sample containing inorganic cyanide is purged with air from a boiling acid solution into a basic
absorber solution. The cyanide salt absorbed in the basic solution is buffered at pH 4.5 and then
reacted with chlorine to form cyanogen chloride. The cyanogen chloride formed couples pyridine
with barbituric acid to form a strongly colored red dye that is proportional to the cyanide
concentration.
This colorimetric method following distillation is the basis for most regulatory methods (for instance
EPA 335.4) used to analyze cyanide in water, wastewater, and contaminated soils.
Distillation followed by colorimetric methods, however, have been found to be prone to
interferences from thiocyanate, nitrate, thiosulfate, sulfite, and sulfide that can result in both positive
and negative bias. It has been recommended by the USEPA (MUR March 12, 2007) that samples
containing these compounds be analyzed by Gas-Diffusion Flow Injection Analysis —
Amperometry.
Gas Diffusion Flow Injection Analysis — Amperometry
Instead of distilling, the sample is injected into an acidic stream where the HCN formed is passed
under a hydrophobic gas diffusion membrane that selectively allows only HCN to pass through.
The HCN that passes through the membrane is absorbed into a basic carrier solution that
transports the CN to an amperometric detector that accurately measures cyanide concentration
with high sensitivity.
Sample pretreatment determined by acid reagents, ligands, or preliminary UV irradiation allow
cyanide speciation of free cyanide, available cyanide, and total cyanide respectively. The relative
simplicity of these flow injection analysis methods limit the interference experienced by the high
heat of distillation and also prove to be cost effective since time consuming distillations are not
required.